Hippocampus as a memory map: synaptic plasticity and memory encoding by hippocampal neurons

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia

This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281-291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675-676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources.

Adverse effect of the combination of hypothyroidism and chronic psychosocial stress on hippocampus-dependent memory in rats

Both hypothyroidism and stress interfere with cognitive function in patients. This study examined the effect of hypothyroidism and stress on hippocampus-dependent learning and memory in rats using the novel radial arm water maze (RAWM), which measures spatial working memory. Hypothyroidism was accomplished by thyroidectomy and 2 weeks later a form of intruder stress was used as the chronic psychosocial stressor. After 4-6 weeks of stress, rats were trained to learn (during the acquisition phase; four trials) and then remember (during two memory test trials occurring 15 and 120 min after the acquisition phase) the within-day location of a hidden escape platform, which was in different arm every day. The number of errors (entry into arms other than the platform arm) was noted. Within-day learning of the platform location was largely unaffected by the experimental manipulations, indicating that rats in all groups were equally capable of finding the platform to escape from the water with similar numbers of errors (P > 0.005). The number of days a rat took to reach a criterion (DTC; a maximum of one error in three consecutive days) indicated that chronic stress or hypothyroidism, alone, resulted in a mild impairment of spatial memory, and the combination of chronic stress and hypothyroidism resulted in a more severe and long-lasting memory impairment. The data indicated that the combination of stress and hypothyroidism produced more deleterious effects on hippocampal function than either chronic stress or hypothyroidism alone.

Hippocampus

The hippocampus serves a critical role in declarative memory--our capacity to recall everyday facts and events. Recent studies using functional brain imaging in humans and neuropsychological analyses of humans and animals with hippocampal damage have revealed some of the elemental cognitive processes mediated by the hippocampus. In addition, recent characterizations of neuronal firing patterns in behaving animals and humans have suggested how neural representations in the hippocampus underlie those elemental cognitive processes in the service of declarative memory.

Frequency-dependent changes in the paired-pulse index in the hippocampus of the freely moving adult male rat

The paired-pulse index (PPI) has been widely used as a measure of modulation of cellular excitability in the hippocampal trisynaptic circuit. This paper presents a quantification of the changes in this measure of neuronal modulation as a result of the application of pulse trains having six different train frequencies (0.1, 1, 5, 8, 15, and 30 Hz) to one of the major efferent pathways to the dentate gyrus, the medial perforant path (MPP). Our findings indicate that the modulation of the first leg of the hippocampal trisynaptic circuit is dependent on the frequency of the "burst train" applied to the perforant pathway. This methodological finding is of importance to all investigators studying hippocampal plasticity via LTP or LTD approaches. The different synaptic mechanisms implicated in being responsible for the changes in the PPI are also discussed.

Categorization in the monkey hippocampus: a possible mechanism for encoding information into memory

The mammalian hippocampus processes sensory information into memory. The neurobiological basis of this representation, as well as the type of information that is encoded, is central to understanding how memories are formed. Normally, there is an infinite amount of information that could be encoded for any given stimulus. Thus, the question arises as to how the hippocampus selects and encodes features of a given stimulus. Here, we show that neurons in the hippocampus of the monkey appear to categorize types of visual stimuli presented in a delayed-match-to-sample memory task. By extracting unique combinations of features, these category cells are able to encode aspects of behaviorally important images instead of encoding all visual details. The subject is then able to rapidly select an appropriate response to that stimulus when distracting stimuli are presented simultaneously, thereby facilitating performance. Moreover, across animals, this specific type of encoding differed considerably. Just as in humans, different monkeys attended to and selected different aspects of the same stimulus image, most likely reflecting different histories, strategies, and expectations residing within individual hippocampal networks.

Impairments in spatial learning and memory: ethanol, allopregnanolone, and the hippocampus

Acute ethanol administration impairs performance in many cognitive tasks that are dependent on hippocampal function. For example, acute ethanol administration produces dose-dependent impairments in spatial learning. Ethanol also decreases the spatial specificity of hippocampal place cells. Such findings raise the possibility that ethanol affects learning and memory by altering, either directly or indirectly, neuronal activity in the hippocampus and related structures. Acute ethanol administration induces a dose- and time-dependent increase in brain concentration of the neuroactive steroid allopregnanolone. Allopregnanolone is a potent GABAA receptor agonist and produces effects similar to the effects produced by ethanol. Blockade of de novo biosynthesis of allopregnanolone alters many of ethanol's effects including ethanol-induced suppression of spontaneous activity in medial septum/diagonal band of Broca neurons and hippocampal pyramidal neurons. These findings suggest that ethanol-induced increases in allopregnanolone levels might play a central role in the effects of acute ethanol on cognitive processing and hippocampal function. The impact of ethanol on spatial cognitive processing and hippocampal function will be reviewed. In addition, the possibility that ethanol-induced changes in neuroactive steroid levels contribute to the impact of ethanol on spatial learning and hippocampal function will be explored.

Extinction, renewal, and spontaneous recovery of a spatial preference in the water maze

Four experiments with C57BL/6 mice investigated extinction of a spatial preference in the Morris water maze. In Experiment 1, a spatial preference was extinguished by exposing mice to the water maze in the absence of a platform but in the presence of the distal spatial cues. In Experiment 2, extinction occurred when the platform was removed from the pool, when it was presented in random locations, or when it was presented consistently in the opposite location. Contextual renewal (Experiment 3) and spontaneous recovery (Experiment 4) of spatial preferences argue against an interpretation of extinction in terms of unlearning and instead suggest that extinction in the water maze, like extinction in Pavlovian conditioning, suppresses the original association. Implications of these findings for theories of spatial learning and hippocampal function are discussed.

Environment-spatial conditional learning in rats with selective lesions of medial septal cholinergic neurons

Cholinergic medial septal neurons may regulate several aspects of hippocampal function, including place field stability and spatial working memory. Monkeys with damage to septal cholinergic neurons are impaired in visual-spatial conditional learning tasks; however, this candidate function of septal cholinergic neurons has not been studied extensively in the rat. In the present study, rats with selective lesions of cholinergic neurons in the medial septum and vertical limb of the diagonal band of Broca (MS/VDB), made with 192 IgG-saporin, were tested on a conditional associative learning task. In this task, which we term "environment-spatial" conditional learning, the correct location of a spatial response depended on the array of local environmental cues. MS/VDB-lesioned rats were impaired when the two parts of the conditional problem were presented concurrently, but not when one environment had been learned before the full conditional problem was presented. Our findings suggest that cholinergic MS/VDB neurons participate in some aspects of conditional associative learning in rats. They may also shed light on the involvement of cholinergic projections to the hippocampus in modulating and remodeling hippocampal spatial representations.

Quantitative morphometry of hippocampal pyramidal cells: Differences between anatomical classes and reconstructing laboratories

The dendritic trees of hippocampal pyramidal cells play important roles in the establishment and regulation of network connectivity, synaptic plasticity, and firing dynamics. Several laboratories routinely reconstruct CA3 and CA1 dendrites to correlate their three-dimensional structure with biophysical, electrophysiological, and anatomical observables. To integrate and assess the consistency of the quantitative data available to the scientific community, we exhaustively analyzed 143 completely reconstructed neurons intracellularly filled and digitized in five different laboratories from 10 experimental conditions. Thirty morphometric parameters, including the most common neuroanatomical measurements, were extracted from all neurons. A consistent fraction of parameters (11 of 30) was significantly different between CA3 and CA1 cells. A considerably large number of parameters was also found that discriminated among neurons within the same morphological class, but reconstructed in different laboratories. These interlaboratory differences (8 of 30 parameters) far outweighed the differences between experimental conditions within a single lab, such as aging or preparation method (at most two significant parameters). The set of morphometrics separating anatomical regions and that separating reconstructing laboratories were almost entirely nonoverlapping. CA3 and CA1 neurons could be distinguished by global quantities such as branch order and Sholl distance. Differences among laboratories were largely due to local variables such as branch diameter and local bifurcation angles. Only one parameter (a ratio of branch diameters) separated both morphological classes and reconstructing laboratories. Compartmental simulations of electrophysiological activity showed that both differences between anatomical classes and reconstructing laboratories could dramatically affect the firing rate of these neurons under different experimental conditions.

Self-organising continuous attractor networks with multiple activity packets, and the representation of space

'Continuous attractor' neural networks can maintain a localised packet of neuronal activity representing the current state of an agent in a continuous space without external sensory input. In applications such as the representation of head direction or location in the environment, only one packet of activity is needed. For some spatial computations a number of different locations, each with its own features, must be held in memory. We extend previous approaches to continuous attractor networks (in which one packet of activity is maintained active) by showing that a single continuous attractor network can maintain multiple packets of activity simultaneously, if each packet is in a different state space or map. We also show how such a network could by learning self-organise to enable the packets in each space to be moved continuously in that space by idiothetic (motion) inputs. We show how such multi-packet continuous attractor networks could be used to maintain different types of feature (such as form vs colour) simultaneously active in the correct location in a spatial representation. We also show how high-order synapses can improve the performance of these networks, and how the location of a packet could be read by motor networks. The multiple packet continuous attractor networks described here may be used for spatial representations in brain areas such as the parietal cortex and hippocampus.

Chromatin remodeling and neuronal response: multiple signaling pathways induce specific histone H3 modifications and early gene expression in hippocampal neurons

Plasticity in gene expression is achieved by a complex array of molecular mechanisms by which intracellular signaling pathways directly govern transcriptional regulation. In addition to the remarkable variety of transcription factors and co-regulators, and their combinatorial interaction at specific promoter loci, the role of chromatin remodeling has been increasingly appreciated. The N-terminal tails of histones, the building blocks of nucleosomes, contain conserved residues that can be post-translationally modified by phosphorylation, acetylation, methylation and other modifications. Depending on their nature, these modifications have been linked to activation or silencing of gene expression. We wanted to investigate whether neuronal stimulation by various signaling pathways elicits chromatin modifications that would allow transcriptional activation of immediate early response genes. We have analysed the capacity of three drugs - SKF82958 (a dopaminergic receptor agonist), pilocarpine (a muscarinic acetylcholine receptor agonist) and kainic acid (a kainate glutamate receptor agonist) - to induce chromatin remodeling in hippocampal neurons. We show that all stimulations induce rapid, transient phosphorylation of histone H3 at serine 10. Importantly, the same agonists induce rapid activation of the mitogen-activated protein kinase pathway with similar kinetics to extracellular-regulated-kinase phosphorylation. In the same neurons where this dynamic signaling cascade is activated, there is induction of c-fos transcription. Histone H3 Ser10 phosphorylation is coupled to acetylation at the nearby Lys14 residue, an event that has been linked to local opening of chromatin structure. Our results underscore the importance of dynamic chromatin remodeling in the transcriptional response to various stimuli in neuronal cells.

Miniature synaptic transmission and BDNF modulate dendritic spine growth and form in rat CA1 neurones

The refinement and plasticity of neuronal connections require synaptic activity and neurotrophin signalling; their specific contributions and interplay are, however, poorly understood. We show here that brain-derived neurotrophic factor (BDNF) increased spine density in apical dendrites of CA1 pyramidal neurones in organotypic slice cultures prepared from postnatal rat hippocampal slices. This effect was observed also in the absence of action potentials, and even when miniature synaptic transmission was inhibited with botulinum neurotoxin C (BoNT/C). There were, however, marked differences in the morphology of individual spines induced by BDNF across these different levels of spontaneous ongoing synaptic activity. During both normal synaptic transmission, and when action potentials were blocked with TTX, BDNF increased the proportion of stubby, type-I spines. However, when SNARE-dependent vesicular release was inhibited with BoNT/C, BDNF increased the proportion of thin, type-III spines. Our results indicate that BDNF increases spine density irrespective of the levels of synaptic transmission. In addition, miniature synaptic transmission provides sufficient activity for the functional translation of BDNF-triggered spinogenesis into clearly defined morphological spine types, favouring those spines potentially responsible for coordinated Ca2+ transients thought to mediate synaptic plasticity. We propose that BDNF/TrkB signalling represents a mechanism of expression of both morphological and physiological homeostatic plasticity in the hippocampus, leading to a more efficient synaptic information transfer across widespread levels of synaptic activity.

Regional variations and age-related changes in nitric oxide synthase and arginase in the sub-regions of the hippocampus

L-arginine can be metabolised by nitric oxide synthase (NOS) with the formation of L-citrulline and nitric oxide (NO), or arginase with the production of L-ornithine and urea. In contrast to studies showing a potential involvement of NOS/NO in the aging process, the role of arginase has not been well documented. The present study investigates for the first time the regional variations and age-related changes in both NOS and arginase in sub-regions of the hippocampus. In young adult rats, although the total NOS activity was not significantly different across the hippocampal CA1, CA2/3 and the dentate gyrus (DG) sub-regions, the total arginase activity showed a clear regional variation with the highest level in DG. Western blotting revealed that the highest levels of neuronal NOS (nNOS) and endothelial NOS (eNOS) proteins were located in CA1. Arginase I is expressed at a very low level in the brain (the whole hippocampus) as compared with the liver. By contrast, arginase II protein shows an extremely high expression in the brain with little or no expression in the liver. There was no regional variation in arginase I or arginase II protein expression across the sub-regions of the hippocampus. When a comparison was made between young (4-month-old) and aged (24-month-old) rats, a significant increase in total NOS activity was found in DG and significant decreases in arginase activity were observed in the CA1 and CA2/3 regions in the aged animals. Western blotting further revealed a dramatic decrease in eNOS protein expression in aged CA2/3 with no age-associated changes in nNOS, arginase I and II protein expression in any region examined. Interestingly, evidence of activity or protein expression of the inducible isoform of NOS (iNOS) was not detected in any tissue from either group. The present results, in conjunction with previous findings, support the contribution of NOS/NO to aging but question the involvement of iNOS in the normal aging process. Region-specific changes in arginase suggest that this enzyme may also contribute to aging.

Glutamate receptors in the rat medial prefrontal cortex regulate set-shifting ability

The authors examined set-shifting abilities in rats injected with antagonists of N-methyl-D-aspartate (NMDA) receptors (MK801) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors (LY293558) into the medial prefrontal cortex (mPFC). Set-shifting was assessed with a maze-based task requiring a switch between brightness and texture discrimination strategies. Intra-mPFC injection of MK801 prior to training on the 2nd discrimination impaired discrimination strategy acquisition. The MK801-induced deficit was due to increased perseverative responding. AMPA receptor blockade also impaired acquisition of the 2nd discrimination; these impairments were due to more general cognitive deficits. Results suggest that, within the mPFC, both AMPA and NMDA receptors are necessary for set-shifting, and that NMDA receptor hypofunction impairs the capacity to modify existing knowledge or to inhibit responses that are no longer appropriate.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30-200 Hz) produces potentiation. In addition, a significant (P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock-Cooper-Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Reward modulates neuronal activity in the hippocampus of the rat

In the hippocampus of freely moving rats, neurons have been recorded that fire predominantly when the animal travels through a particular area while exploring the environment (so-called 'place cells'). This study investigates if the neuronal firing characteristics of such cells are modulated by attention, expectation of reward or memory load. A total of 16 electrodes were implanted in the CA1 region of the hippocampus of 3-month-old Long-Evans rats. Using a tetrode recording system, single neurons were recorded while a rat explored an 8-arm maze and retrieved pellets at the end of each arm. It was found that 31 out of 67 neurons showed place cell characteristics, while the other cells either fired in more than one place or fired along whole arms of the maze. Interestingly, 11 of the 31 neurons showed enhanced firing activity when the animal entered a baited arm but did not fire when the arm was visited again after the bait had been retrieved. In a second experiment, only four out of eight arms were baited. Firing rates of 46 neurons were analysed, and all cells (spatial or non-spatial) fired more in baited arms than in non-baited ones (P<0.001). In a reversal task in which the previously unbaited four arms were subsequently baited, neuronal activity was increased in the newly baited arms (42 cells analysed, P<0.001). Since no alterations to the maze or cues have been made, we interpret the increased firing probability of neurons in baited arms compared to unbaited arms as a correlate for 'attention' or 'expectation'.

Involuntary, unreinforced (pure) spatial learning is impaired by fimbria-fornix but not by dorsal hippocampus lesions

Pure spatial learning occurs when rats acquire information about an environment while exploring it in the absence of reinforcers. We previously reported that voluntary, unreinforced exploration of a radial maze retards subsequent reinforced conditioned cue preference (CCP) learning in the same maze. In the present experiment, we examined the effects of involuntary, unreinforced pre-exposure to a radial maze. During pre-exposure, rats were moved by an experimenter between the ends of two arms of a radial maze five times in 30 min. This form of pre-exposure retarded CCP learning, whereas rats that were not pre-exposed and rats that were pre-exposed to a maze in a different room displayed normal CCP learning. These findings suggest that some information specific to the maze environment was acquired during involuntary unreinforced pre-exposure to it. In experiment 2, the retardation of reinforced CCP learning by involuntary unreinforced pre-exposure was eliminated by fimbria-fornix lesions made before pre-exposure but was unaffected by fimbria-fornix lesions made after pre-exposure but before training. Large neurotoxic lesions of the dorsal hippocampus made before pre-exposure had no effect on the retardation of CCP learning, but the rats with these lesions were impaired on a standard test of reinforced spatial learning in a water maze. The lesion effects in experiment 2 are similar to those previously reported for voluntary exploration and suggest that pure spatial learning may occur during both voluntary exploration of and involuntary exposure to an environment in the absence of reinforcers. Pure spatial learning can apparently occur with exposure to two different locations within an environment, but the rats do not have to move between the locations voluntarily. An intact fimbria-fornix is required for acquisition but not expression of this form of learning. The hippocampus is not involved in this form of learning.

Oxytocin improves long-lasting spatial memory during motherhood through MAP kinase cascade

Oxytocin is an essential hormone for mammalian labor and lactation. Here, we show a new function of oxytocin in causing plastic changes in hippocampal synapses during motherhood. In oxytocin-perfused hippocampal slices, one-train tetanus stimulation induced long-lasting, long-term potentiation (L-LTP) and phosphorylation of cyclic AMP-responsive element binding protein (CREB), and MAP kinase inhibitors blocked these inductions. An increase in CREB phosphorylation and L-LTP induced by one-train tetanus were observed in the multiparous mouse hippocampus without oxytocin application. Furthermore, intracerebroventricular injection of oxytocin in virgin mice improved long-term spatial learning in vivo, whereas an injection of oxytocin antagonist in multiparous mice significantly inhibited the improved spatial memory, L-LTP and CREB phosphorylation. These findings indicate that oxytocin is critically involved in improving hippocampus-dependent learning and memory during motherhood in mice.

Spatial memory performance of freely-moving squirrel monkeys

Few experiments have addressed the problem of cognitive map formation in non-human primates. Therefore, a paradigm was developed to assess spatial memory formation in squirrel monkeys (Saimiri sciureus) moving freely in three dimensions. While moving on the walls and floor of a large test chamber, the animals learned to collect pieces of cereal from baited food-ports interspersed among non-baited ports. The cereal-pellets were not visible to the monkeys, so the animals needed to develop spatial memory to visit only the baited ports for food and avoid the non-baited ones. A session consisted of ten consecutive trials, and 3 successive sessions were conducted on each day for a 5-day period. For each trial, correct choices (CC; number of visited baited-ports) and incorrect choices (IC; number of visited non-baited ports) were registered, and spatial memory performance index (SMPI; ranging from 0.00 to 1.00) was calculated as follows: SMPI=(CC-IC)/CC. For each session, mean SMPI, session duration, total reaches into the non-baited ports, and total reaches into the baited ports were documented. In an 8-port task, where 4 food-ports were baited and 4 were non-baited, the mean SMPI was higher than 0 in the first session (day 1), indicating the development of short-term spatial memory. By the fifth session (day 2), this index was significantly higher than in the first session, indicating the build-up of long-term spatial memory. These changes were related to a significant decrease in the total reaches into the non-baited ports. At the same time, the duration of the sessions and the total reaches into the baited ports did not change significantly. This paradigm can be used for (1) studying cognitive map formation in primates, (2) examining the underlying cellular and molecular mechanisms in integrative neurobiological experiments, and (3) screening cognition-enhancer drugs in a monkey model.

Molecular and cellular cognitive studies of the role of synaptic plasticity in memory

Synaptic plasticity has a central role in nearly all models of learning and memory. Besides experiments documenting changes in synaptic function during learning, most of the evidence supporting a role for synaptic plasticity in memory comes from manipulations that either enhance or lesion synaptic processes. In the last decade, mouse transgenetics (knock outs and transgenics) have provided compelling evidence that the molecular mechanisms responsible for the induction and stability of synaptic changes have a critical role in the acquisition and storage of information. Here, I will review this literature, with a special focus on studies of hippocampal-dependent learning and memory.

Impaired Performance of Fornix-Transected Rats on a Distal, but Not on a Proximal, Version of the Radial-Arm Maze Cue Task

Fornix-transected and sham-operated rats were trained on radial maze cue tasks in which the relative positions of the cues were either fixed (F condition) or varied (V condition) across trials. Proximal and distal visual stimuli were used in 2 different experiments. With proximal stimuli, fornix-transected rats were transiently impaired in the V condition and performed as well as controls in the F condition. However, using extramaze stimuli, fornix-transected rats were severely impaired in the V condition but performed normally in the F condition. According to histological analyses, performance on these cue tasks varied along with the extent of cholinergic depletion in the hippocampus. At the behavioral level, the location and stability of stimuli's relative positions seemed to have influenced rats' performance.

Time, Space and Hippocampal Functions

The hippocampus is one of the most researched structures of the brain. Studies of lesions in humans, primates and rodents have suggested to some that the primary role of the hippocampus is to act as a temporary memory buffer which is required for the consolidation of long-term memory. The famous case study of patient H.M., in particular, seemed to suggest that the hippocampus was of crucial importance for memory formation. However, recordings of single neurons in freely moving rodents did not support this notion. In such recordings, neurons were found that were active predominately when the animal passed through a particular area in space. Consequently, these neurons were termed 'place cells' and a theory was developed that suggested that the hippocampus acts as a 'cognitive map' that is required for spatial orientation. It was then found that H.M. had significant damage to his temporal lobes that included the amygdala, rhinal cortices, and other areas. Further case studies and selective hippocampal lesions in primates resulted in much milder amnestic symptoms, and lesions of defined cortical areas in the temporal lobes showed that a number of functions previously attributed to the hippocampus were in fact linked to these areas. Further analysis of neuronal activity in the hippocampus showed that not only is spatial information represented there, but also additional information, such as speed of movement, direction of movement, match or non-match detection, olfactorial identification, and others. In addition, it was found that selective lesions of the hippocampus in rodents impaired spatial navigation and memory formation only mildly. Only simultaneous lesions of several cortical areas in conjunction with the hippocamus could reproduce the impairments and symptoms that were previously thought to be observed after hippocampal lesions alone. In conclusion it is proposed that information processing and memory formation is shared by several brain areas that act as a functional system. This review presents evidence from many different studies that the hippocampus is part of this system and plays a supportive role in associating complex multimodal information and laying down new memory traces. In addition, the concept of allocating specific functions (such as the development of a cognitive map) exclusively to the hippocampus is rejected.

Impairment of L-type Ca2+ channel-dependent forms of hippocampal synaptic plasticity in mice deficient in the extracellular matrix glycoprotein tenascin-C

The extracellular matrix glycoprotein tenascin-C (TN-C) has been suggested to play important functional roles during neural development, axonal regeneration, and synaptic plasticity. We generated a constitutively TN-C-deficient mouse mutant from embryonic stem cells with a floxed tn-C allele, representing a standard for future analysis of conditionally targeted mice. The gross morphology of the CNS was not detectably affected, including no evidence for perturbed nerve cell migration, abnormal oligodendrocyte distribution, or defective myelination. Despite the apparent normal histology of the hippocampus and normal performance in the water maze, theta-burst stimulation (TBS) of Schaffer collaterals elicited reduced long-term potentiation (LTP) in the CA1 region of TN-C-deficient mutants, as compared with wild-type littermates. However, high-frequency stimulation evoked normal LTP not only in CA1, but also at mossy fiber-CA3 and medial and lateral perforant path-granule cell synapses in the dentate gyrus. Low-frequency stimulation failed to induce long-term depression in the CA1 region of TN-C-deficient animals. Recordings of TBS-induced LTP in the presence of nifedipine, an antagonist of L-type voltage-dependent Ca2+ channels (VDCCs), did not affect LTP in TN-C-deficient mice, but reduced LTP in wild-type mice to the levels seen in mutants. Furthermore, chemical induction of a L-type VDCC-dependent LTP in the CA1 region by application of the K+ channel blocker tetraethylammonium resulted in impaired LTP in TN-C mutants. Thus, reduction in L-type VDCC-mediated signaling appears to mediate the deficits in certain forms of synaptic plasticity in constitutively TN-C-deficient mice.

Properties of the extra-positional signal in hippocampal place cell discharge derived from the overdispersion in location-specific firing

There is a good deal of evidence that in the rodent, an internal model of the external world is encoded by hippocampal pyramidal cells, called 'place cells'. During free exploration, the activity of place cells is higher within a small part of the space, called the firing field, and virtually silent elsewhere. We have previously shown that the spiking activity during passes through the firing field is characterized not only by the high firing rate, but also by its very high variability ('overdispersion'). This overdispersion indicates that place cells carry information in addition to position. Here we demonstrate by simulations of an integrate-and-fire neuronal model that while a rat is foraging in an open space this additional information may arise from a process that alternatingly modulates the inputs to place cells by about 10% with a mean period of about 1 s. We propose that the overdispersion reflects switches of the rats attention between different spatial reference frames of the environment. This predicts that the overdispersion will not be observed in rats that use only room-based cues for navigation. We show that while place cell firing is overdispersed in rats during foraging in an open arena, the firing is less overdispersed during the same behavior in the same environment, when the rats have been trained to use only room-based and not arena-based cues to navigate.

Blind mice are not impaired in T-maze footshock avoidance acquisition and retention

The processing of visual information during learning and memory is considered to be a vital function of the hippocampus. Some researchers believe that the sole purpose of the hippocampus is to process visuo-spatial information, whereas other investigators believe that the hippocampus integrates cues from multiple sources. In the current studies, we tested the effects of vision loss on a hippocampal task, acquisition and retention with T-maze footshock avoidance conditioning. Acquisition and retention, in adult-blinded mice, were not significantly impaired in T-maze footshock avoidance. Blindness did not affect activity, footshock startle or motivation to avoid shock. The same doses of memory enhancing drugs that improve memory in sighted mice improved memory in blind mice. Electrolytic lesions in blind mice, which destroyed 31+/-4% of the hippocampus, significantly impaired acquisition and retention for T-maze footshock avoidance and so demonstrated that the hippocampus retained its integrative role in blind mice. The current findings show that blind mice are as capable of learning T-maze footshock avoidance as sighted mice and that the hippocampus retains its important role in blind mice in learning and memory processing. It is concluded that the T-maze footshock avoidance conditioning task is a spatially but not visually dependent task that is hippocampally dependent.

MK-801 disrupts acquisition of contextual fear conditioning but enhances memory consolidation of cued fear conditioning

The effects of pre-training or post-training subcutaneous injections of multiple doses of the non-competitive NMDA-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) on cued and contextual fear conditioning were examined in F344 rats. Pre-training injections of MK-801 (0.3 and 1.0 mg/kg) disrupted contextual fear conditioning but not cued fear conditioning. Post-training injections of MK-801 did not disrupt cued or contextual fear conditioning. In fact, the 0.3 mg/kg dose of MK-801 enhanced cued fear conditioning. Finally, rats were tested for MK-801-induced alterations in sensitivity to pain using the formalin test for nociception. MK-801 did not reduce sensitivity to pain. These results suggest that NMDA receptors are involved in acquisition of contextual fear conditioning but not in memory consolidation of the learned response.

Model systems for assessing cognitive function: implications for HIV-1 infection and drugs of abuse

Memory deficits are common among drug abusers and in those with chronic neurodegenerative disorders. Currently, the mechanisms through which diverse neurophysiologic processes alter memory are not known. This review describes the current systems and rationale for studying memory formation, consolidation, and recall. Special attention is given to physiologic (hippocampal long-term potentiation) and behavioral animal models. The principles and methods described can be applied to studies of diverse clinical disorders.

Rats can track odors, other rats, and themselves: implications for the study of spatial behavior

In order to demonstrate that rats solve dead reckoning (path integration) tasks in which they return to a starting location using self-movement (idiothetic) cues, it is necessary to remove external (allothetic) cues. Odor cues, especially those generated by a rat on a single passage, are difficult to control and they can potentially serve as a cue to guide a homeward trip. Because it is presently unknown whether rats can track the cues that they themselves leave, as opposed to the odor trails left by other rats, we investigated this question in the present study. A tracking task was used in which rats: (1) followed a scented string from a refuge to obtain a food pellet located on a large circular table; (2) followed odors left on the table; (3) followed odors left by the passage of another rat; or (4) followed odors left by themselves. Groups of rats were presented with strings scented with either the rat's own odor (Group Own), a conspecific's odor (Group Other), or another scent, vanilla (Group Vanilla). After training, a series of discrimination tests were given to determine the nature of the stimulus that controls scent tracking. The results indicated that Own, Other, and Vanilla groups were equally proficient in discriminating and following their respective odors. The rats were also able to follow odor trails on the table surface as well as a trail left by the single passage of another rat or their own passage. This is the first study to demonstrate that rats can discriminate between conspecific odors and their own odor left during a single passage. The results are discussed in relation to their implications for experimental methodology and olfactory contributions to spatial navigation in general and dead reckoning in particular.

Impaired dodging in food-conflict following fimbria-fornix transection in rats: a novel hippocampal formation deficit

It is well known that damage to the hippocampal formation (Ammon's horn, dentate gyrus, fimbria-fornix, and other pathways) produces impairments in spatial navigation and in certain forms of learning. Lesions within these structures have also been reported to produce some motor impairments, but the nature of these impairments is less understood. The present study examined the effects of fimbria-fornix lesions on food wrenching and dodging, social interactions that occur when one rat attempts to steal food from a conspecific, who in turn attempts to protect the food by an evasive movement. Lesion effectiveness was confirmed histologically and electrophysiologically, by the loss of hippocampal rhythmical slow-wave activity (RSA or theta), and by changes in open field behavior (increased open field behavior, less thigmotaxis and more defecation). Analysis of the social interaction indicated when an eating control rat was approached by a conspecific that was attempting to steal its food, it prevented the theft by dodging, a rapid lateral maneuver involving forequarter turning and stepping with the rear limbs. Rats with fimbria-fornix lesions were significantly impaired in dodging and so were more likely to lose their food to the robber. This novel deficit in motor behavior is discussed in relation to contemporary theories of hippocampal function and it is suggested that the deficit may be caused by an inability of the fimbria-fornix damaged animals to disengage attention from eating in order to initiate an evasive movement to protect food. The finding of this novel deficit underscores the importance of considering both loss as well as release phenomena in the analysis of hippocampal formation function.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

The hippocampal formation--orbitomedial prefrontal cortex circuit in the attentional control of active memory

The long held view that the hippocampal formation is not only essential, but also solely responsible for declarative memory in humans (and by analogy non-human primates) has come into question. Based on extensive reciprocal connection patterns between the hippocampal formation and the orbitoventromedial prefrontal cortex in primates and rats, a central role for the hippocampal formation in the attentional control of behavior is emerging. In this paper, evidence is reviewed showing that the hippocampal-orbitomedial prefrontal cortex circuit may be involved in attentional monitoring of the internal sensorium. This attentional monitoring system, in a sense, is the working memory of viscero-emotional processing. The hippocampal formation can thus be viewed as a discrepancy detector with respect to the relative activational status of cognitive/emotional set in the orbitomedial prefrontal cortex. Discrepancies between the current representation of the internal milieu and the "just-prior" representation held "on-line" in orbitomedial prefrontal cortex associative working memory, are signaled from the hippocampus to the prefrontal cortex prospective attentional systems to activate, process, and reconcile internal (past) with external (present) environments, and finally to effectively alter active working emotional "sets" to exert cognitive-emotional control of behavior.

Dead reckoning (path integration) requires the hippocampal formation: evidence from spontaneous exploration and spatial learning tasks in light (allothetic) and dark (idiothetic) tests

Animals navigate using cues generated by their own movements (self-movement cues or idiothetic cues), as well as the cues they encounter in their environment (distal cues or allothetic cues). Animals use these cues to navigate in two different ways. When dead reckoning (deduced reckoning or path integration), they integrate self-movement cues over time to locate a present position or to return to a starting location. When piloting, they use allothetic cues as beacons, or they use the relational properties of allothetic cues to locate places in space. The neural structures involved in cue use and navigational strategies are still poorly understood, although considerable attention is directed toward the contributions of the hippocampal formation (hippocampus and associated pathways and structures, including the fimbria-fornix and the retrosplenial cortex). In the present study, using tests in allothetic and idiothetic paradigms, we present four lines of evidence to support the hypothesis that the hippocampal formation plays a central role in dead reckoning. (1) Control but not fimbria-fornix lesion rats can return to a novel refuge location in both light and dark (infrared) food carrying tasks. (2). Control but not fimbria-fornix lesion rats make periodic direct high velocity returns to a starting location in both light and dark exploratory tests. Control but not fimbria-fornix rats trained in the light to carry food from a fixed location to a refuge are able to maintain accurate outward and homebound trajectories when tested in the dark. (3). Control but not fimbria-fornix rats are able to correct an outward trajectory to a food source when the food source is moved when allothetic cues are present. These, tests of spontaneous exploration and foraging suggest a role for the hippocampal formation in dead reckoning.

The hippocampal debate: are we asking the right questions?

For years, the debate has been: "Is the hippocampus the cognitive map?" or "Is the hippocampus the core of memory?" These two hypotheses derived their original power from two key experiments--the cognitive map theory from the remarkable spatial correlates seen in recordings of hippocampal pyramidal cells and the memory theory from the profound amnesias seen in the patient H.M. Both of these key experiments have been reinterpreted over the years: hippocampal cells are correlated with much more than place and H.M. is missing much more than just his hippocampus. However, both theories are still debated today. The hippocampus clearly plays a role in both navigation and memory processing. The question that must be addressed is rather: "What is the role played by the hippocampus in the navigation and memory systems?" By looking at the navigation system as a whole, one can identify the major role played by the hippocampus as correcting for accumulation errors that occur within idiothetic navigation systems. This is most clearly experimentally evident as reorientation when an animal is lost. Carrying this over to a more general process, this becomes a role of recalling a context, bridging a contextual gap, or, in other words, it becomes a form of recognition memory. I will review recent experimental data which seems to support this theory over the more general spatial or memory theories traditionally applied to hippocampus.

Cerebral microvascular pathology in aging and Alzheimer's disease

The aging of the central nervous system and the development of incapacitating neurological diseases like Alzheimer's disease (AD) are generally associated with a wide range of histological and pathophysiological changes eventually leading to a compromised cognitive status. Although the diverse triggers of the neurodegenerative processes and their interactions are still the topic of extensive debate, the possible contribution of cerebrovascular deficiencies has been vigorously promoted in recent years. Various forms of cerebrovascular insufficiency such as reduced blood supply to the brain or disrupted microvascular integrity in cortical regions may occupy an initiating or intermediate position in the chain of events ending with cognitive failure. When, for example, vasoconstriction takes over a dominating role in the cerebral vessels, the perfusion rate of the brain can considerably decrease causing directly or through structural vascular damage a drop in cerebral glucose utilization. Consequently, cerebral metabolism can suffer a setback leading to neuronal damage and a concomitant suboptimal cognitive capacity. The present review focuses on the microvascular aspects of neurodegenerative processes in aging and AD with special attention to cerebral blood flow, neural metabolic changes and the abnormalities in microvascular ultrastructure. In this context, a few of the specific triggers leading to the prominent cerebrovascular pathology, as well as the potential neurological outcome of the compromised cerebral microvascular system are also going to be touched upon to a certain extent, without aiming at total comprehensiveness. Finally, a set of animal models are going to be presented that are frequently used to uncover the functional relationship between cerebrovascular factors and the damage to neural networks.

The effect of interburst intervals on measures of hippocampal LTP in the freely moving adult male rat

An important factor in the induction and maintenance of long-term potentiation (LTP) is the tetanization paradigm. This paper presents the changes associated with the induction and maintenance of hippocampal LTP in the freely moving adult male rat, subjected to three different tetanization paradigms. These results indicate that specific LTP measures including (1) synaptic activation, as measured by the slope of the dentate granule cell population excitatory postsynaptic potential, and (2) cellular response, as measured by the dentate population spike amplitude, evoked by single-pulse stimulation of the medial perforant pathway are dependent on the interburst interval of the bursting paradigm commonly used in LTP studies.

Instability in the place field location of hippocampal place cells after lesions centered on the perirhinal cortex

The perirhinal cortex appears to play a key role in memory, and the neighboring hippocampus is critically involved in spatial processing. The possibility exists, therefore, that perirhinal-hippocampal interactions are important for spatial memory processes. The purpose of the present study was to investigate the contribution of the perirhinal cortex to the location-specific firing ("place field") of hippocampal complex-spike ("place") cells. The firing characteristics of dorsal CA1 place cells were examined in rats with bilateral ibotenic acid lesions centered on the perirhinal cortex (n = 4) or control surgeries (n = 5) as they foraged in a rectangular environment. The activity of individual place cells was also monitored after a delay period of either 2 min, or 1 or 24 hr, during which time the animal was removed from the environment. Although the perirhinal cortex lesion did not affect the place field size or place cell firing characteristics during a recording session, it was determined that the location of the place field shifted position across the delay period in 36% (10 of 28) of the cells recorded from lesioned animals. In contrast, none of the place cells (0 of 29) recorded from control animals were unstable by this measure. These data indicate that although the initial formation of place fields in the hippocampus is not dependent on perirhinal cortex, the maintenance of this stability over time is disrupted by perirhinal lesions. This instability may represent an erroneous "re-mapping" of the environment and suggests a role for the perirhinal cortex in spatial memory processing.

Hippocampal dependent learning ability correlates with N-methyl-D-aspartate (NMDA) receptor levels in CA3 neurons of young and aged rats

Hippocampal N-methyl-D-Aspartate (NMDA) receptors mediate mechanisms of cellular plasticity critical for spatial learning in rats. The present study examined the relationship between spatial learning and NMDA receptor expression in discrete neuronal populations, as well as the degree to which putative age-related changes in NMDA receptors are coupled to the effects of normal aging on spatial learning. Young and aged Long-Evans rats were tested in a Morris water maze task that depends on the integrity of the hippocampus. Levels of NR1, the obligatory subunit for a functional NMDA receptor, were subsequently quantified both biochemically by Western blot in whole homogenized hippocampus, and immunocytochemically by using a high-resolution confocal laser scanning microscopy method. The latter approach allowed comprehensive, regional analysis of discrete elements of excitatory hippocampal circuitry. Neither method revealed global changes, nor were there region-specific differences in hippocampal NR1 levels between young and aged animals. However, across all subjects, individual differences in spatial learning ability correlated with NR1 immunofluorescence levels selectively in CA3 neurons of the hippocampus. Parallel confocal microscopic analysis of the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptor failed to reveal reliable differences as a function of age or spatial learning ability. This analysis linking age, performance, and NR1 levels demonstrates that although dendritic NR1 is generally preserved in the aged rat hippocampus, levels of this receptor subunit in selective elements of hippocampal circuitry are linked to spatial learning. These findings suggest that NMDA receptor abundance in CA3 bears a critical relationship to learning mediated by the hippocampus throughout the life span.

Disparate effects of long-term potentiation on evoked potentials and single CA1 neurons in the hippocampus of anesthetized rats

To examine the effects of long-term potentiation (LTP) on individual neurons in the intact brain, anesthetized rats were implanted with a recording stereotrode in the right CA1 layer of the hippocampus and a stimulating electrode in the right and left CA3 layers. The evoked and spontaneous firing of single CA1 neurons was characterized before and after LTP of the contralateral (commissural) Schaffer collaterals and again after LTP of the ipsilateral (associational) Schaffer collaterals. Individual CA1 neurons displayed either increases or decreases in evoked and spontaneous firing after LTP. As many as five discriminated cells were recorded simultaneously, and they typically responded discordantly, so that after LTP, firing in some neurons increased while in others it decreased. The response of individual neurons to in vivo LTP may be modulated by heterogeneous synaptic changes on individual and local groups of cells, and by changes in feed-forward excitation and inhibition provided by local hippocampal circuitry.

Involvement of the 5-HT1A receptors in classical fear conditioning in C57BL/6J mice

The present study examined the involvement of the 5-HT(1A) receptors in classical fear conditioning using the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propyloamino)tetralin hydrobromide (8-OH-DPAT) and the selective "silent" 5-HT(1A) receptor antagonist (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexane carboxamide trihydrochloride (WAY 100635). The drugs were administered both subcutaneously and bilaterally into the dorsal hippocampus of male C57BL/6J mice. The training was performed in a single trial in which a tone was followed by a footshock. The retention of context- and tone-dependent fear was examined in separate tests conducted either 1 or 24 hr after training. Subcutaneous 8-OH-DPAT (0.1-1.0 mg/kg), when injected before but not after training, caused a dose-dependent impairment of contextual fear in both 1 and 24 hr tests, whereas tone-dependent fear was less affected. Pretraining intrahippocampal injections of 5.0 microg but not 1.0 microg 8-OH-DPAT caused a severe deficit in contextual fear when tested 24 hr after training. When injected both subcutaneously and intrahippocampally, 8-OH-DPAT induced the 5-HT syndrome, indicative of postsynaptic 5-HT(1A) receptor activation at the dose ranges that impaired fear conditioning. However, the behavioral changes induced by 8-OH-DPAT at the time of training could not account for inhibitory effects of 8-OH-DPAT on fear conditioning. Neither subcutaneous (0.03 mg/kg) nor intrahippocampal (0.5 microg per mouse) WAY 100635 altered context- or tone-dependent fear. However, subcutaneous WAY 100635 blocked both the 5-HT syndrome and the impairment of fear conditioning induced by subcutaneous or intrahippocampal 8-OH-DPAT. In contrast, intrahippocampal WAY 100635 blocked the impairment caused by intrahippocampal but not subcutaneous 8-OH-DPAT, indicating the involvement of extrahippocampal 5-HT(1A) receptors in fear conditioning. It is concluded that the deficits in fear conditioning induced by 8-OH-DPAT are a result of postsynaptic 5-HT(1A) receptor activation that interferes with learning processes operating at acquisition but not consolidation. Furthermore, the dorsohippocampal 5-HT(1A) receptors play an important but not exclusive role in the limbic circuitry subserving contextual fear conditioning.

Understanding hippocampal activity by using purposeful behavior: place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.

Understanding hippocampal activity by using purposeful behavior: Place navigation induces place cell discharge in both task-relevant and task-irrelevant spatial reference frames

Continuous rotation of an arena in a cue-rich room dissociates the stationary room-bound information from the rotating arena-bound information. This disrupted spatial discharge in the majority of place cells from rats trained to collect randomly scattered food. In contrast, most place cell firing patterns recorded from rats trained to solve a navigation task on the rotating arena were preserved during the rotation. Spatial discharge was preserved in both the task-relevant stationary and the task-irrelevant rotating reference frames, but firing was more organized in the task-relevant frame. It is concluded that, (i) the effects of environmental manipulations can be understood with confidence only when the rat's purposeful behavior is used to formulate interpretations of the data, and (ii) hippocampal place cell activity is organized in multiple overlapping spatial reference frames.